It is well known in the art that the coating on a workpiece can be measured by utilizing X-ray fluorescence and varying machines have been developed for this purpose. Representative patents showing the state of the art are U.S. Pat. Nos. 4,406,015 and 4,656,357. The prior art typically includes a frame upon which is mounted an X-ray tube and signal detector. The X-ray beam is projected from the tube through a shutter, and then through a collimator to the workpiece, the surface of which must be held at a fixed distance from the X-ray tube and the detector. In order to properly position the workpiece with respect to the X-ray tube and collimator, a fixed focal length viewing device is typically employed, which device may be a microscope, or a closed circuit television camera and monitor. The image of the surface of the workpiece seen through the viewing device is reflected by a mirror located in the path of the X-ray beam in order to avoid parallax, the mirror being either provided with a small aperture as shown in 4,656,357 so that the X-rays can pass through the mirror, or being transparent to the X-ray beam as taught by 4,406,015. The workpiece is mounted upon a holder which can be adjusted in x, y and z axes to properly position the coating on the workpiece at the fixed distance away from the X-ray tube where the surface is in focus when viewed through the viewing device. While this form of device has proven to be generally satisfactory in most situations it has been found that when measuring coatings on very small areas it is not always possible to achieve satisfactory results. This is because the X-ray beam which exits from the terminal end of the collimator will spread outwardly in a slightly conical projection. Thus, if a very small area is to be measured and if the workpiece is disposed at some distance below the mirror, the X-ray beam which contacts the workpiece will be spread beyond to the sides of the area to be measured thereby preventing accurate measurement of the coating thickness.